Pyroelectric materials are known for use as thermal detectors. The internal electrical polarisation of a pyroelectronic material changes rapidly with temperature in the region of its Curie temperature. Single-element pyroelectric detectors have been used as infrared detectors in burglar alarms, radiometers and the like. The pyroelectric material is arranged as a capacitor dielectric, and an electrical signal across the capacitor is monitored for changes arising from dielectric property variation with temperature affected by infrared absorption. Reticulated multi-element arrays of pyroelectric detectors are also known, in for example the pyroelectric vidicon tube. In this device the pyroelectric material is scanned with an electron beam to read out the varying surface charge pattern induced by infrared emission from a scene. The electrical current in the tube circuit is dependant on the polarisation charge on the detector surface at the point addressed by the electron beam.
A pyroelectric material for a thermal detector is selected to have its Curie temperature at about ambient temperature to permit operation without cooling. Sensitivity is a function of pyroelectric properties. Unfortunately, no improvement in performance can be obtained by cooling; performance can in fact worsen with cooling if the material is taken far from its Curie point. Compounds with suitable pyroelectric properties are comparatively expensive to make in the form of detector quality slices of pyroelectric material suitable for detector arrays. Moreover, the required signal read-out apparatus is complex. The pyroelectric vidicon tube for example requires a comparatively bulky vacuum tube containing an electron gun and an arrangement of beam deflection and current collection electrodes. The electron beam produces an undesirable degree of electrical noise.
Generally speaking, photon detectors are more sensitive than pyroelectric thermal detectors. However, for high sensitivity applications cooling to cryogenic temperatures (77.degree. K.) is required, resulting in an undesirably bulky, complex and expensive arrangement. Simple thermoelectric cooling devices of acceptable power requirements are not adequate to give the necessary degree of cooling. Joule-Kelvin effect coolers are required. Photovoltaic devices such as photodiodes and Schottky photoemitters are known, these being usually operated at low and stable temperature. They accordingly normally consist of comparatively thick semiconductor material in intimate thermal contact with a heat sink and cooler. This minimizes and maintains constant unwanted thermally induced transitions.